Subcutaneous Implantation Medical Device With Multiple Parasternal-Anterior Electrodes

ABSTRACT

A subcutaneous implantable medical device and method (SIMD) provided. A pulse generator (PG) is configured to be positioned subcutaneously within a lateral region of a chest of a patient. The PG has a housing that includes a PG electrode. The PG has an electronics module. An elongated lead is electrically coupled to the pulse generator. The elongated lead includes a first electrode that is configured to be positioned along a first parasternal region proximate a sternum of the patient and a second electrode that is configured to be positioned at an anterior region of the patient. The first and second electrodes are coupled to be electrically common with one another. The electronics module is configured to provide electrical shocks for antiarrhythmic therapy along at least one shocking vector between the PG electrode and the first and second electrodes.

BACKGROUND

Embodiments of the present disclosure relate generally to subcutaneousimplantable medical devices and methods, and more particularly tomedical devices having pulse generators and leads that are implantedsubcutaneously.

Currently, implantable medical devices (IMD) are provided for a varietyof cardiac applications. IMDs may include a “housing” or “canister” (or“can”) and one or more electrically-conductive leads that connect to thecanister through an electro-mechanical connection. IMDs may containelectronics (e.g., a power source, microprocessor, capacitors, etc.)that control electrical activation of the leads to provide variousfunctionalities. For instance, the IMD may be configured for pacemaking,cardioversion, and/or defibrillation. An implantablecardioverter-defibrillator (ICD) is one such medical device and it isdesigned to monitor heart rate, recognize certain events (e.g.,ventricular fibrillation or ventricular tachycardia), and deliverelectrical shock to reduce the risk of sudden cardiac death (SCD) fromthese events. The ICD may be used for patients who have alreadyexperienced potentially life-threatening events or for those that are atrisk of SCD. The ICD includes a pulse generator and one or more leadshaving electrodes that may be used to detect how the heart isfunctioning or provide electrical shock to the heart.

One type of ICD delivers therapy through transvenous leads that areadvanced to the right ventricle for detection and treatment oftachyarrhythmia. Transvenous ICDs (or TV-ICDs) may also providebradycardia-pacing support. Although TV-ICDs can be helpful and preventsudden cardiac death, TV-ICDs may have certain drawbacks or potentialcomplications. For instance, it can be difficult and time-consuming toachieve venous access, thereby prolonging the medical procedure. TV-ICDscan be associated with hemopericardium, hemothorax, pneumothorax, leaddislodgement, lead malfunction, device-related infection, and venousocclusion. Transvenous leads may also malfunction through conductorfailure in the leads or breaches in the insulation that surrounds theconductors.

A second type of ICD, referred to as a subcutaneous ICD (or S-ICD), usesan electrode configuration that can reside entirely within thesubcutaneous space. The pulse generator is positioned along a side ofthe patient's chest below the arm pit (e.g., over the sixth rib near theleft mid-axillary line). A lead extends from the pulse generator alongthe side of the patient toward the sternum. The lead then turns toextend parallel to the mid-stemal line and is positioned adjacent to thestemum extending between the xiphoid process and the manubriostemaljunction. This portion of the lead includes a shock coil that is flankedby two sensing electrodes. The sensing electrodes sense the cardiacrhythm and the shock coil delivers counters-hocks through thesubcutaneous tissue of the chest wall. Unlike the transvenous types, theS-ICDs lack intravenous and intracardiac leads and, as such, are lesslikely to have the noted complications associated with more invasivedevices. Current electrode configurations for S-ICDs, however, have somechallenges or undesirable features. For instance, conventionalcommercially available as ICDs are relatively large and exhibit higherdefibrillation threshold (DFTs), as compared to modern transvenouSIMDs.For example, a conventional S-ICD may be 60-70 mL in volume, as comparedto a 30 mL transvenouSIMD. As another example, a conventional S-ICD mayutilize DFTs of 80 J, as compared to 40 J for transvenouSIMDs. A desireremains to further improve upon the size and energy demands of S-ICDs.

SUMMARY

In accordance with embodiments herein, a subcutaneous implantablemedical device (SIMD) is provided. A pulse generator (PG) is configuredto be positioned subcutaneously within a lateral region of a chest of apatient. The PG has a housing that includes a PG electrode. The PG hasan electronics module. An elongated lead is electrically coupled to thepulse generator. The elongated lead includes a first electrode that isconfigured to be positioned along a first parastemal region proximate asternum of the patient and a second electrode that is configured to bepositioned at an anterior region of the patient. The first and secondelectrodes are coupled to be electrically common with one another. Theelectronics module is configured to provide electrical shocks forantiarrhythmic therapy along at least one shocking vector between the PGelectrode and the first and second electrodes.

Optionally, the anterior region may represent a second parastemalregion. The second electrode may be configured to be oriented to extendin a common direction with the first electrode and along a midline ofthe sternum. The first and second electrodes may be configured to bespaced apart by a distance of between 1.5 cm and 8 cm as measured in adirection transverse to a midline of the sternum. The first and secondelectrodes may be configured to be positioned along opposite sides ofthe stemum and separated from one another by a predetermined distance.The first and second electrodes may be configured to be positioned on acommon side of the sternum and separated from one another by apredetermined distance. At least one of the first and second electrodesmay have an active length of 7-11 cm and the predetermined distanceseparating the first and second electrodes may be 6.0 to 8.5 cm.

Optionally, the anterior region may be located proximate to a lower endof the sternum. The second electrode may be configured to be oriented toextend in a non-parallel direction relative to a direction of the firstelectrode. The second electrode may be located at relative to a midlineof the sternum that may be vertically below the first electrode. Thefirst electrode may have a length that is greater than a length of thesecond electrodes. The length of the first electrode may be 7.0 cm to11.0 cm, and the length of the second electrode may be 5.0 cm to 6.5 cm.The lead may include a lead body having a base segment with a proximalend to be electrically coupled to a header of the PG.

Optionally, the lead may include first and second distal branches at adistal end of the lead. The first and second electrodes may be providedon the first and second distal branches, respectively.

The lead body may further comprise a Y-connector connecting the firstand second distal branches to a distal end of the base segment. Thepulse generator and first and second electrodes may define a shockingvector that may provide to a defibrillation threshold of at most 20Joules. The pulse generator and first and second electrodes may define ashocking vector there between that may exhibit an impedance along theshocking vector that is no more than 64 ohms. The pulse generator andfirst and second electrodes may define a shocking vector there betweenthat may exhibit an impedance along the shocking vector that is no morethan 56.2 ohms.

In accordance with embodiments herein, a method is provided. The methodcomprises implanting a pulse generator (PG) within a lateral region of achest of a patient. The PG has a housing that includes a PG electrode.The method implants at least one lead having first and second electrodesthat are coupled to be electrically common with one another. The firstand second electrodes are elongated. The implanting comprisingpositioned the first electrode along a first parastemal region proximateto a sternum of the patient and positioning the second electrode at ananterior region of the patient. The method configures the electronicsmodule to provide electrical shocks for antiarrhythmic therapy along atleast one shocking vector between the PG electrode and the first andsecond electrodes.

Optionally, the method may comprise positioning the first and secondelectrodes at a dual parastemal position extending in a common directionand spaced apart by a distance of 1.5 cm to 8.5 cm. The method maycomprise positioning the first and second electrodes along oppositesides of the sternum and positioning the first and second electrodes ona common side of the sternum. At least one of the first and secondelectrodes may have an active length of 8-11 cm and the predetermineddistance separating the first and second electrodes may be 6.0 to 8.5cm. The method may comprise positioning the second electrode proximateto a lower end of the sternum and orienting the second electrode toextend in a non-parallel direction relative to a direction of the firstelectrode, locating the second electrode at a position, relative to amidline of the sternum that may be vertically below the first electrode.The non-parallel direction may orient a longitudinal axis of the secondelectrode perpendicular to a longitudinal axis of the first electrode.The pulse generator may be configured to generate a defibrillatingenergy of at most 40 Joules.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a graphical representation of a subcutaneousimplantable medical system that is configured to apply therapy to aheart in accordance with embodiments herein.

FIG. 1B illustrates a graphical representation of a subcutaneousimplantable medical system that is configured to apply therapy to aheart in accordance with embodiments herein.

FIG. 1C illustrates an SIMD formed in accordance with embodimentsherein.

FIG. 1D illustrates an SIMD formed in accordance with embodimentsherein.

FIG. 1E illustrates an SIMD formed in accordance with embodimentsherein.

FIG. 2 illustrates a simple block diagram of at least a portion of thecircuitry within the SIMD in accordance with embodiments herein.

FIG. 3A illustrates a graphical representation of a conventionalconfiguration that includes a pulse generator and an electrode inaccordance with embodiments herein.

FIG. 3B illustrates graphical representations of SIMD configurationsimplemented in accordance with embodiments herein.

FIG. 3C illustrates graphical representations of SIMD configurationsimplemented in accordance with embodiments herein.

FIG. 3D illustrates graphical representations of SIMD configurationsimplemented in accordance with embodiments herein.

FIG. 3E illustrates graphical representations of SIMD configurationsimplemented in accordance with embodiments herein.

FIG. 3F illustrates graphical representations of SIMD configurationsimplemented in accordance with embodiments herein.

FIG. 4 illustrates a flow chart of a method for implanting asubcutaneous implantable medical system in accordance with embodimentsherein.

FIG. 5 illustrates a block diagram of an SIMD in accordance withembodiments herein.

DETAILED DESCRIPTION

Embodiments set forth herein include implantable medical devices (SIMDs)and methods of using and positioning the same. In particularembodiments, the SIMD includes a subcutaneous implantablecardioverter-defibrillator (S-ICD). Embodiments include a pulsegenerator that is positioned within a lateral region of a chest of apatient. The PG has a housing or canister that includes a PG electrode.Embodiments also include at least one lead having first and secondelectrodes with the first electrode positioned along a first parastemalregion of the chest of the patient and the second electrode positionedalong a second anterior region of the chest.

As used herein, the term “subcutaneously,” when used to describeimplanting a device (e.g., pulse generator, lead body, electrode, etc.),means implanting the device beneath the skin but above layers ofskeletal muscle tissue, rib bones, and costal cartilage. The device istypically positioned under the subcutaneous tissue. When the term“subcutaneous” is used to characterize the entire implantable medicalsystem, the term means that most of the operating components of thesystem (e.g., the pulse generator, shocking electrodes, optional sensingelectrodes, lead bodies) or each and every one of the operatingcomponents is beneath the skin, but above layers of skeletal muscletissue, rib bones, and costal cartilage. Compared to transvenous ICDimplantation, subcutaneous implantation may be less complex, lessinvasive, and less time-consuming. In some embodiments, however, one ormore components may not be subcutaneous.

The terms “vertical”, “horizontal” and “lateral”, as used in connectionwith describing electrode orientation and position, are used generallyrelative to a longitudinal axis extending through a patient chest andmore specifically relative to an axis extending along a midline of asternum of the patient's check. For example, the sternum shall beconsidered to extend in a “vertical” direction, regardless of whetherthe patient is in a standing, sitting, prone or other position. Asfurther examples, the horizontal and lateral directions referinterchangeably to a direction that is perpendicular to the midline ofthe sternum.

Embodiments herein provide an SIMD that exhibits defibrillationthresholds (DFTs) that are less than conventional SIMDs. For example,the DFT in some embodiments is at most 22 Joules, and in other certainembodiments is at most 20 Joules or, more particularly, 10-20 Joules.

The terms “the lead” and “at least one lead” shall include only a singlelead and more than one single lead. For the avoidance of doubt, when“the lead” or “at least one lead” is described to include first andsecond electrodes, it is understood that the first and second electrodesmay be provided as a common single lead body or the first and secondelectrodes may be provided on different first and second lead bodies offirst and second leads, respectively.

FIGS. 1A and 1B illustrate a graphical representation of a subcutaneousimplantable medical system that is configured to apply therapy to aheart. FIG. 1A illustrates a torso of a patient to show the rib cage anda general outline of the heart and greater vessels. In particularembodiments, the system may apply high voltage defibrillation shocks, aswell as other general arrhythmia therapy, such as pacing therapy,cardiac resynchronization therapy (CRT), and the like. The systemincludes a subcutaneous implantable medical device (SIMD) 14 that isconfigured to be implanted in a subcutaneous area exterior to the heart.The system includes only the SIMD and is entirely or fully subcutaneous.As shown in FIG. 1A, the SIMD 14 is positioned within a lateral region,such as along the left side of the rib cage under the left arm. The SIMD14 may be positioned relative to a vertical direction substantiallyaligned with the apex of the heart. The SIMD 14 is configured to delivervarious arrhythmia therapies, such as defibrillation therapy, pacingtherapy, antitachycardia pacing therapy, cardioversion therapy, and thelike. The system does not require insertion of includes a transvenouslead. It is contemplated, however, that system may include othercomponents. For example, alternative embodiments may include atransvenous lead or a leadless electrode in addition to the structuresin FIG. 1A.

The lead 20 includes at least two electrodes 22, 24 that are used forproviding electrical shock for defibrillation. Optionally, the lead 20may include one or more sensing electrodes. The pulse generator 15 maybe implanted subcutaneously and at least a portion of the lead 20 may beimplanted subcutaneously. In particular embodiments, the SIMD 14 is anentirely or fully subcutaneous SIMD. The pulse generator 15 may bepositioned at a lateral position or below an apex of the heart.

With reference to FIG. 1B, the lead 20 includes an elongated lead body60 that extends from a proximal end 62 to a distal tip 64. The pulsegenerator 15 includes a housing 18 that is configured to be active toform a pulse-generator (PG) electrode 19. The pulse generator 15 alsoincludes a header 17 mounted to the housing 18. The header 17 isconfigured to receive and be connected to the proximal end 62 of thelead body 60. The proximal end 62 may include one or more contacts (notshown) that electrically engage respective terminals (not shown) in theheader 17 of the pulse generator 15.

The elongated lead body 60 may be formed as a flexible tube or sleeve 66comprising, for example, a blocompatible material (e.g., polyurethane,silicone, etc.). The lead body 60 may include a single lumen (orpassage) or multiple lumen (or passages) within the flexible tube 66.The lead 20 also includes a plurality of electrical conductors (notshown) that extend along the lumen and electrically couple theelectrodes 22, 24 (and optionally sensing electrodes) to the pulsegenerator 15. The electrical conductors may be cabled conductors coatedwith PTFE (poly-tetrafluoroethylene) and/or ETFE(ethylenetetrafluoroethylene). The lead body 60 may be configured forreceiving a stylet that enables positioning of the lead. The electricalconductors are terminated to the respective electrodes. For example, theconductors may be terminated to respective electrodes of the proximalend 62 and then respective electrodes 22, 24 (described below).

The lead body 60 may include one or more distal branches 21, 23 thatseparate from a splitting connector 25, where the distal branches 21, 23each include a corresponding one of the electrodes 22, 24. As shown, thelead body 60 includes two distal branches 21, 23 and two electrodes 22,24, although it is recognized that more than two branches and more thantwo electrodes may be provided on the lead body 60. Additionally oralternatively, two or more separate leads 20 may be provided, with eachlead 20 having a single distal segment and single electrode providedthereon. For example, the electrodes 22 and 24 may be provided onseparate leads that are individually joined to the header 17. The distalbranches 21, 23 may have a common length or different lengths. Forexample, the distal branch 23 may be longer than the distal branch 21,such as when the electrode 24 is intended to be implanted at a locationfurther from the pulse generator 15, as compared to the distance betweenthe pulse generator 15 and the electrode 22. The splitting connector 25may be configured in different shapes and different manners. Forexample, the splitting connector 25 may be formed as a Y-connector, aT-connector and the like. The splitting connector 25 may be formed aspart of a monotonic unitary body structure with the lead body 60 anddistal branches 21, 23. Optionally, the connector 25 may be formed aspart of a monotonic unitary body structure with the lead body 60, butseparate from the distal branches 21 and 23. For example, the distalbranches 21 and 23 may include connector pins at the proximal endsthereof, where the connector pins are configured to be received intoterminals within the connector 25. Optionally, the connector 25 may beformed as part of a monotonic unitary body structure with the distalbranches 21, 23 and include a connector pin or terminal at the proximalend that is configured to join a mating terminal or connector pin on thedistal end of the lead body 60. Providing a connection at the connector25 enables different combinations of lead bodies 60 and distal branches21, 23 to accommodate different implant locations and orientations,different shocking vectors and different anatomic dimensions.

The electrodes 22, 24 may be referred to as first and second electrodes22, 24 that are coupled to be electrically common with one another. Thefirst and second electrodes 22, 24 are elongated along correspondinglongitudinal axes. During the implantation operation the implantingcomprises positioned the first electrode 22 along a first parastemalregion proximate to a sternum of the patient and positioning the secondelectrode 24 at an anterior region of the patient. The first and secondelectrodes 22, 24 may be positioned in a dual parastemal combinationextending in a common direction and spaced apart by a distance of 1.5 cmto 8.5 cm. The positioning operation may comprise positioning the firstand second electrodes 22, 24 along opposite sides of the sternum, orpositioning the first and second electrodes 22, 24 on a common side ofthe sternum. At least one of the first and second electrodes 22, 24 hasan active length of 8-11 cm. A predetermined spacing separates the firstand second electrodes. The anterior positioning operation may comprisepositioning the second electrode proximate to a lower end of the stemumand orienting the second electrode to extend in a direction non-parallelto a direction of the first electrode, and locating the second electrodeat a position, relative to a midline of the stemum, that is verticallybelow the first electrode. The non-parallel direction may orient alongitudinal axis of the second electrode perpendicular to alongitudinal axis of the first electrode.

With reference to FIG. 1A, the first electrode 22 may be positionedalong a left side of the anterior region of the chest adjacent to thesternum. The second electrode 24 may be positioned along a right side ofthe anterior region of the chest adjacent to the sternum. The distalbranches 21, 23 diverge from one another at the connector 25 to enablethe electrodes 22, 24 to be spaced apart from one another, onceimplanted, with an electrode gap 74 there between that has apredetermined spacing such as 1.5 cm to 8.5 cm. The electrode gap 74represents a spacing between the electrodes 22, 24 when the electrodes22, 24 are implanted generally parallel to and along opposite sides ofthe sternum. For example, the electrodes 22, 24 may be spaced equaldistances from a midline (FIG. 1A) extending vertically through apatient along a center of the sternum. Optionally, one of the electrodes22, 24 may be positioned laterally closer to the sternum and midline,while the other of the electrodes 22, 24 is positioned laterally furtheraway from the sternum and midline. The electrodes 22, 24 may bepositioned subcutaneously to extend vertically along a region adjacentopposite sides of the sternum that aligns with the heart of the patientfor providing a sufficient amount of energy for defibrillation.

FIG. 1C illustrates an SIMD 114 formed in accordance with an alternativeembodiment. The SIMD 114 includes a pulse generator 115 coupled to alead 120. The lead 120 includes a lead body 121 that includes a distalportion 137 with a distal electrode 124. The distal electrode 124 iscoupled through a distal branch 123 of the lead body 121 to anintermediate electrode 122. The distal branch 123 interconnects theelectrodes 124 and 122 in a generally U-shaped alignment when theelectrodes 122, 124 are implanted along a generally common implantdirection.

In the example of FIG. 1C, a graphical illustration is presented to showan example stemum 125 that extends along a midline 127. The electrodes122, 124 may be located on opposite sides of the sternum 125 andoriented parallel to the midline 127. The electrodes 122, 124 are spacedapart by a predetermined spacing 135 and are implanted to be orientedvertically in a common direction with the sternum 125. The electrodes122, 124 may be shifted left or right by various degrees relative to themidline 127 of the sternum 125. The electrodes 122, 124 are spacedlateral distances 129, 131, respectively, from the midline 127. Thelateral distances 129, 131 may be equal to one another and/or differfrom one another. For example, the electrode 122 may be positionedfurther away from the midline 127 of the sternum 125, as compared to thespacing of the electrode 124, such that the lateral distance 129 isgreater than the lateral distance 131. As another example, the electrode124 may be positioned further away from the midline 127 of the sternum125, as compared to the spacing of the electrode 122, such that thelateral distance 131 is greater than the lateral distance 129. Theelectrode 122 may join the lead body 121 at an intermediate connection133 in various manners. For example, the intermediate connection 133 maybe formed as a Y connection, T connection and the like.

FIG. 1D illustrates an SIMD 154 formed in accordance with an alternativeembodiment. The SIMD 154 includes a pulse generator 155 coupled to alead 170. The lead 170 includes a lead body 171 having first and secondelectrodes 172 and 174 formed in a monotonic unitary manner therewith.The lead 170 is formed with the first electrode 172 at an intermediatepoint along the lead body 171 in an in-line arrangement with the secondelectrode 174. The first electrode 172 includes a proximal end thatjoins the base segment of the lead body 171. The first electrode 172includes a distal end that joins an intermediate segment 173 of the leadbody 171. The intermediate segment 173 interconnects the first andsecond electrodes 172 and 174 in a generally straight line.

During an implant operation, the lead 170 is inserted such that thefirst electrode 172 is located generally parallel to a midline 177 ofthe sternum 175 and spaced a first distance from the midline 177, whilethe second electrode 174 is also generally parallel to the midline 177of the stemum and spaced a second distance from the midline 177. Thefirst and second electrodes 172 and 174 are spaced apart from oneanother by a predetermined spacing 185 (e.g., 1.5 cm to 8.5 cm). Thefirst and second electrode 172 and 174 may be located both on a leftside, both on a right side or on opposite sides of the sternum 175. Thelead 170 is bent such that the first and second electrodes 172 and 174,and the intermediate segment 173 form generally a U-shape.

FIG. 1E illustrates an SIMD formed in accordance with an alternativeembodiment. The SIMD includes a pulse generator 180 connected to a lead182. The lead 182 includes a lead body 181 having a proximal endconnected to a header of the PG 180. The lead body 181 includes asplitting connector 183 located at an intermediate point along a lengthof the lead body 181. The lead 182 includes a parastemal branch 185 andan anterior branch 186 that extend from the splitting connector 183.Distal ends of the parastemal and anterior branches 185 and 186 includefirst and second electrodes 182 and 184, respectively. The first andsecond electrodes 182 and 184 may also be referred to as a parastemalelectrode and an anterior electrode, respectively, to be indicative ofthe implant locations of the electrodes. The first and second electrodes182 and 184 are elongated to extend along corresponding longitudinalaxes and have corresponding active lengths 190 and 191. The firstelectrode 182 may have an active length of 7.0 cm to 11.0 cm, and morespecifically between 8.0 cm and 10.0 cm. The second electrode 184 mayhave an active length of 5.0 cm to 6.5 cm, and more specifically between5.8 cm and 6.2 cm. However, it is understood that the active lengths ofthe first and second electrodes 182 and 184 may vary based on variouscriteria.

During an implant operation, the first (parastemal) electrode 182 ispositioned at a parastemal location proximate to a left or right side ofthe sternum 192 and spaced a lateral spacing 194 from a midline 193 ofthe stemum 192. The first electrode 182 is oriented to extend generallyin a common direction (e.g., parallel) to the midline 193. The firstelectrode 182 may be shifted vertically up or down along a verticaldirection 189 along the sternum 195 to a desired vertical positionrelative to the heart. The parastemal branch 185 extends from thesplitting connector 183 to a proximal end of the first electrode 182.

The second (anterior) electrode 184 is positioned at an anteriorlocation proximate to a lower tip of the sternum 192. The secondelectrode 184 second electrode 184 is positioned relative to the firstelectrode 182, such as spacing the second electrode 184 a verticalspacing 187 from a lower/proximal end of the first electrode 182.Optionally, the second electrode 184 may be positioned relative to alower tip of the sternum such as spacing the second electrode 184 avertical spacing below the lower tip of the sternum. The secondelectrode 184 is shifted left/right in a lateral direction 188 to adesired position relative to one or both of the midline 193 of thesternum 192 and/or a longitudinal axis of the first electrode 182.

FIG. 2 illustrates a simple block diagram of at least a portion of thecircuitry within the SIMD 14. The SIMD 14 includes a controller 30 thatmay be coupled to cardiac sensing circuitry 32 and pulse sensingcircuitry 34. The controller 30 also utilizes or communicates withvarious other electronic components, firmware, software, and the likethat generally perform sensing and pacing functions (as generallydenoted by a pacemaker functional block 36). While the examples hereinare provided for pacing and defibrillation functions, the SIMD could beprogrammed to perform anti-tachycardia pacing, cardiac rhythm therapy,and the like. The cardiac sensing circuitry 32 is configured to detectcardiac events. The pulse sensing circuitry 34 is configured to detectevent markers.

The controller 30 is configured to analyze incoming paced cardiac events(as sensed over the cardiac sensing circuitry 32). Based on thisanalysis, the controller 30 in the SIMD 14 may perform various pacemakerrelated actions, such as setting or ending timers, recording data,delivery of therapy, and the like. The controller 30 of the SIMD 14 mayalso perform various cardioversion/defibrillation related functions. Inthe example of FIG. 2, outputs 38 and 40 represent output terminals thatare coupled through a switching circuit (in the functional module 36) tocorresponding electrodes on the housing of the SIMD 14. Alternatively,the outputs 38 and 40 may be coupled to respective electrodes on alongthe lead 20 (FIG. 1).

Inputs 42-48 are provided to the cardiac and pulse sensing circuitry 32and 34. By way of example, with reference to SIMD 14, inputs 42 and 44may be coupled to sensing electrodes that supply sensed signals to asensing amplifier 52. Inputs 46 and 48 may be coupled to the same ordifferent sensing electrodes to provide sensed signals to a pulseamplifier 54. An output of the sensing amplifier 52 is supplied toamplitude discriminator 56, while an output of the pulse amplifier 54 issupplied to amplitude discriminator 58. Outputs of the amplitudediscriminators 56 and 58 are then provided to the controller 30 forsubsequent analysis and appropriate actions. The inputs 42 and 44 may becoupled to various combinations of the electrodes 22, 24 or the PGelectrode 19.

FIG. 3A illustrates a graphical representation of a conventionalconfiguration that includes a pulse generator 305 and an electrode 302.The pulse generator 305 is located at an anterior position along theleft side of the rib cage under the left arm and proximate to an apex ofthe heart 304. The electrode 302 has a 10 cm active length and ispositioned in a frontal chest region proximate the stemum 303 and on theleft side of the sternum 303.

FIGS. 3B-3F illustrate graphical representations of SIMD configurationsimplemented in accordance with embodiments herein. FIG. 3B illustrates anew configuration with the pulse generator 305 located at an anteriorposition along the left side of the rib cage under the left arm (similarto the conventional configuration of FIG. 3A). Unique to the newembodiment of FIG. 3B, dual parastemal electrodes 310 and 312 arelocated on opposite left/right sides of the sternum 303 and spaced apartfrom one another by 7.0 cm. The electrodes 310 and 312 each includeactive lengths of 10 cm.

FIG. 3C illustrates a new configuration with the pulse generator 305located at an anterior position along the left side of the rib cageunder the left arm. Unique to the new embodiment of FIG. 3C, dualparastemal electrodes 314 and 316 are both located on the right side ofthe sternum 303 and spaced apart from one another by 2.0 cm. Theelectrodes 314 and 316 each include active lengths of 10 cm.

FIG. 3D illustrates a new configuration with the pulse generator 305located at an anterior position along the left side of the rib cageunder the left arm. Unique to the new embodiment of FIG. 3D, dualparastemal electrodes 318 and 320 are both located on the left side ofthe sternum 303 and spaced apart from one another by 2.0 cm. Theelectrodes 318 and 320 each include active lengths of 8 cm.

FIG. 3E illustrates a new configuration with the pulse generator 305located at an anterior position along the left side of the rib cageunder the left arm. Unique to the new embodiment of FIG. 3D, aparastemal electrode 322 is located on the right side of the sternum 303in combination with an anterior electrode 324 that is located below thesternum 303. The anterior electrode 324 is spaced apart 2 cm below alower end of the parastemal electrode 322. The parastemal and anteriorelectrodes 322 and 324 are oriented generally perpendicular to oneanother in an L-shape. The electrodes 322 and 324 include active lengthsof 10 cm and 6 cm, respectively.

FIG. 3F illustrates a new configuration with the pulse generator 305located at an anterior position along the left side of the rib cageunder the left arm. Unique to the new embodiment of FIG. 3F, aparastemal electrode 326 is located on the left side of the sternum 303in combination with an anterior electrode 328 that is located below thesternum 303. The anterior electrode 328 is spaced apart 2 cm below theparastemal electrode 326. The parastemal and anterior electrodes 326 and328 are oriented generally perpendicular to one another in an L-shape.The electrodes 326 and 328 include active lengths of 10 cm and 6 cm,respectively.

The conventional configuration of FIG. 3A and the new configurations ofFIGS. 3B-3F were modeled through computer simulations to identifystimulation characteristics. Table 1 below illustrates the results ofthe computer simulations, including defibrillation thresholds (DFTs) inJoules (J), current defibrillation thresholds (IDFTs) in amps (A),impedance (Z) in ohms, and PT(V) 0.02% mass. In table 1, row/case #1corresponds to the conventional configuration described above inconnection with FIG. 3A. The computer simulations indicated that theDFTs would be 24 jewels, DFT current 10.2 A, impedance 68 ohms and PT(V) 30.

TABLE 1 PT (V) Case DFT IDFT Z 0.02% # Configurations (J) (A) (ohms)Mass 1 10 cm sternal coil (left) 24 10.2 68 30 to CAN (Control) 2 10 cmsternal dual coils 12 9.8 50.5 28 R&L (7 cmsp) to CAN 3 10 cm sternaldual coils 14.7 9 60 31 2 cmsp Right −> CAN 4 8 cm sternal dual coils 2010 64 28 2 cmsp Left −> CAN 5 10 cm sternal R + ant 6 cm 1 cmsp(L-shaped) 16 10.1 56.2 31 to CAN 6 10 cm sternal Left + ant 21.7 12.254 30 6 cm 2 cmsp (L-shaped) to CAN

Row/case #2 corresponds to a new dual parastemal configuration describedabove in connection with FIG. 3B. The computer simulations indicatedthat, when dual parastemal 10 cm active coil electrodes are located onthe right and left sides of the sternum at a 7 cm spacing, the DFTswould be 12 joules, DFT current 9.8 A, impedance 50.5 ohms and PT (V)28. Row/case #3 corresponds to another new dual parastemal configurationdescribed above in connection with FIG. 3C. The computer simulationsindicated that, when dual parastemal 10 cm active coil electrodes arelocated both on the right side of the stemum at a 2 cm spacing, the DFTswould be 14.7 joules, DFT current 9.0 A, impedance 60 ohms and PT (V)31. Row/case #4 corresponds to another new dual parastemal configurationdescribed above in connection with FIG. 3D. The computer simulationsindicated that, when dual parastemal 8 cm active coil electrodes arelocated both on the left side of the sternum at a 2 cm spacing, the DFTswould be 20.0 joules, DFT current 10.0 A, impedance 64 ohms and PT (V)28.

Row/case #5 corresponds to a new combination parastemal-anteriorconfiguration described above in connection with FIG. 3E. The computersimulations indicated that, when a parastemal 10 cm active coilelectrode is located on the right side of the sternum and a 6 cm activecoil electrode is located at an anterior position spaced 1 cm below theparastemal electrode, the DFTs would be 16.0 joules, DFT current 10.1 A,impedance 56.2 ohms and PT (V) 31. Row/case #6 corresponds to anothercombination parastemal—anterior configuration described above inconnection with FIG. 3F. The computer simulations indicated that, when aparastemal 10 cm active coil electrode is located on the left side ofthe sternum and a 6 cm active coil electrode is located at an interiorposition spaced 2 cm below the parastemal electrode, the DFTs would be21.7 joules, DFT current 12.0 A, impedance 54 ohms and PT (V) 30.

From Table 1, it can be seen that the new configurations proposed inaccordance with embodiments herein provide substantial unexpectedimprovements over the conventional configuration. For example, theconfiguration of case 2 (FIG. 3B) defines a shocking vector thatprovides a substantially lower defibrillation threshold (12 J v 24 J) ascompared to the conventional configuration (case 1). The reduction indefibrillation thresholds was due in part to a substantial decrease inthe impedance experienced across the shocking vector (50.5 ohms v 68ohms) between the electrodes and the PG housing. As further examples,cases 3 and 8 (corresponding to FIGS. 3C and 3E) define shocking vectorsthat provide substantially lower defibrillation thresholds (14.7 J and16 J) as compared to the defibrillation threshold of the conventionalconfiguration (24 J in case 1). Again, the reduction in DFTs was in partdue to a reduction in the impedance experienced across the shockingvectors (60 ohms and 56.2 ohms, respectively) between the housing of theSIMD and the combination of electrodes (e.g., dual parastemal orparastemal in case 3 and anterior in case 8).

FIG. 4 illustrates a flow chart of a method 400 for implanting asubcutaneous implantable medical system in accordance with anembodiment. The subcutaneous implantable medical system may be similaror identical to the subcutaneous implantable medical systems describedherein. The method 400 includes implanting, at 402, a pulse generator(PG) subcutaneously within a lateral region of the chest, such as alongthe left side of the rib cage under the left arm. For example, anincision is made to place the pulse generator at the mid-axillary linebetween the 5^(th) and 6^(th) intercostal spaces. The SIMD 14 may bepositioned relative to a vertical direction substantially aligned withthe apex of the heart. The SIMD 14 is configured to deliver variousarrhythmia therapies, such as defibrillation therapy, pacing therapy,antitachycardia pacing therapy, cardioversion therapy, and the like. ThePG has a housing that includes a PG electrode.

At 405, a pocket may be formed. The pocket may be subcutaneous orsubmuscular. The pocket may be completed before or after positioning thelead as described below. For subcutaneous pockets, the subcutaneoustissue may be dissected or separated from the pectoral fascia using adesignated tool (e.g., knife). In some embodiments, an inflatableballoon may be inserted through the incision and inflated to displacethe subcutaneous tissue. For submuscular pockets, the transverse musclefibers of the pectoralis major may be separated for providing access infront of the pectoralis minor. The pulse generator may be positionedwithin the pocket before or after lead placement.

The method also includes implanting, at 406, at least one lead havingfirst and second electrodes. Access incisions may be used to manipulatethe lead for placing the first and second electrodes at designatedpositions. The implanting, at 406, may include making an access incisionat a xiphoid location, at 408 and, optionally, making one or two otheraccess incisions at 410. The access incision made at 408 may beproximate to a bottom of the stemum. The access incision(s) made at 410may be located near a top of the stemum (e.g., superior incisions). Fromthe xiphoid incision, two additional tunnels are made to the superiorincisions that can be on either the left or right check or on both theleft and right chest for parastemal coil electrodes.

Implanting, at 406, may also include tunneling, at 412, a tract betweenthe different incisions and advancing the lead through the tract. Forexample, a tunneling device (e.g., elongated tube) may be insertedthrough the first access incision. For example, at 408, the first accessincision may be located at a point near a bottom of the sternum along aside of the sternum where the electrode is to be positioned (e.g., onthe left or right side of the sternum). The tunneling device separatesthe subcutaneous tissue from underlying tissue (e.g., muscle, bone)between the first access incision and the pocket in the lateral regionbelow the left arm. When an end of the tunneling device is accessiblethrough the pocket, a distal end of the lead may be coupled to the endof the tunneling device (e.g., through a suture loop). The tunnelingdevice is then withdrawn from the first access incision, at 414, therebypulling the lead through the newly-formed tract between the pocket andthe first access incision. In accordance with some embodiments, thesplitting connector (e.g., Y-connector) will be used to connect twoparastemal coil electrodes at the xiphold incision to the lead body.

Previously or subsequently, another tunneling device may be insertedthrough a second access incision (at 410), such as at a point near anupper end of the stemum and on the corresponding left or right side ofthe sternum where the distal end of the parastemal electrode is toterminate. At 412, the tunneling device is advanced downward along apath generally parallel to the sternum toward the first access incisionalong a corresponding side of the sternum. When the end of the tunnelingdevice is accessible through the first access incision, the lead may becoupled to the end of the tunneling device. The tunneling device is thenwithdrawn from the second access incision, thereby pulling the leadthrough the newly-formed tract between the first access incision and thesecond access incision. This process may be repeated as necessary forpositioning a second parastemal electrode on the same or an oppositeside of the sternum and/or for an anterior electrode located below thesternum. It is understood that for an anterior electrode, the firstaccess incision may be sufficient to pull the interior electrode to thefinal termination point, without the need for a second access incision.

Optionally, when using dual parastemal electrodes, the pair ofparastemal electrodes may be pulled through the first access incision atthe same time, such as when formed as part of a single lead body.Optionally, when separate leads are used for the dual parastemalelectrodes, the separate leads may be pulled through a common firstaccess incision at the same time and/or through separate first andsecond access incisions at different times. After positioning the firstand second electrodes, the pulse generator may be connected to the lead.More specifically, an end portion of the lead with contacts may beinserted into one or more terminal ports on a header of the pulsegenerator.

At 418, a defibrillation test may be performed to determine adefibrillation threshold. The defibrillation threshold is a quantitativeestimate of the ability of the heart to defibrillate. The defibrillationthreshold is typically defined as the minimum shock strength that causesdefibrillation. The defibrillation threshold can be measured by changingthe voltages in subsequent VF inductions in accordance with apredetermined protocol. For example, the stored voltages may beincrementally decreased for subsequent VF inductions until the firstshock is unable to defibrillate. If a high defibrillation threshold isidentified, it may be desirable to make adjustments to the system. Forexample, the lead could be repositioned, the lead could be switched-out,portions of the electrodes could be capped, or another lead may beadded.

FIG. 5 illustrates a block diagram of an SIMD 601. The SIMD 601 iscapable of performing stimulation therapy, including cardioversion,defibrillation, and pacing stimulation. The SIMD 601 is hereinafterreferred to as the device 610. While a particular multi-element deviceis shown, this is for illustration purposes only. It is understood thatthe appropriate circuitry could be duplicated, eliminated or disabled inany desired combination to provide a device capable of monitoringimpedance and/or cardiac signals, and/or treating the appropriatechamber(s) with cardioversion, defibrillation and pacing stimulation.

The housing 640 for the device 610 is often referred to as the“canister,” “can,” “case,” or “case electrode” and may be programmablyselected to act as the shock electrode and/or as a return electrode forsome or all sensing modes. The housing 640 may further be used as areturn electrode alone or in combination with one or more otherelectrodes. The housing 640 further includes a connector (not shown)having a plurality of terminals 647-652. To achieve sensing, pacing, andshocking in connection with desired chambers of the heart, the terminals647-652 are selectively connected to corresponding combinations ofelectrodes.

The device 610 includes a programmable microcontroller 660 that controlsthe various modes of sensing and stimulation therapy. Themicrocontroller 660 includes a microprocessor, or equivalent controlcircuitry, designed specifically for controlling sensing impedancederivation and the delivery of stimulation therapy and may furtherinclude RAM or ROM memory, logic and timing circuitry, state machinecircuitry, and I/O circuitry. The microcontroller 660 includes theability to process or monitor input signals (data) as controlled by aprogram code stored in memory. The details of the design and operationof the microcontroller 660 are not critical to the present invention.Rather, any suitable microcontroller 660 may be used.

The microcontroller 660 includes inputs that are configured to collectcardiac signals associated with electrical or mechanical behavior of aheart over at least one cardiac cycle. The cardiac signals may be fromthe cardiac sensing circuit 682 and representative of electricalbehavior of the heart. The circuit 682 may provide separate, combined,composite or difference signals to the microcontroller 660representative of the sensed signals from the electrodes. Optionally,the cardiac signals may be the output of the A/D circuit 690 that arerepresentative of electrical behavior of the heart. The cardiac signalsmay be the output of the physiologic sensor 607 that are representativeof mechanical behavior.

The microcontroller 660 includes a cardiac signal (CS) module 661, amarker detection (MD) module 663 and a therapy module 665 (among otherthings). The CS module 661 is configured to analyze cardiac signals. TheMD module 663 is configured to analyze signals sensed over the markersensing channel and identify incoming event markers. The therapy module665 is configured to modulate, over multiple cardiac cycles: at leastone therapy parameter while the device 610 obtains a collection of atleast one CSF indicators associated with different therapy parameters.The therapy module 665 is further configured to adjust a therapyconfiguration based on, among other things, the cardiac signals andbased on the event markers.

The microcontroller 660 further controls a shocking circuit 617 by wayof a control signal. The shocking circuit 617 generates stimulatingpulses of low (up to 0.5 Joules), moderate (0.5-10 Joules), or highenergy (11 to 50 Joules), as controlled by the microcontroller 660.Stimulating pulses may be applied to the patient's heart through atleast two shocking electrodes.

One or more pulse generators 670 and 672 generate various types oftherapy, such as pacing and ATP stimulation pulses for delivery bydesired electrodes. The electrode configuration switch 674 (alsoreferred to as a switch bank) controls which terminals 647-652 areconnected to the pulse generators 670, 672, thereby controlling whichelectrodes receive a therapy. The pulse generators, 670 and 672, mayinclude dedicated, independent pulse generators, multiplexed pulsegenerators, shared pulse generators or a single common pulse generator.The pulse generators 670 and 672 are controlled by the microcontroller660 via appropriate control signals to trigger or inhibit stimulationpulses. The microcontroller 660 further includes timing controlcircuitry which is used to control the timing of such stimulation pulses(e.g., pacing rate, atrio-ventricular (AV) delay, atrial interconduction(A-A) delay, or ventricular interconduction (V-V) delay, etc.) as wellas to keep track of the timing of refractory periods, PVARP intervals,noise detection windows, evoked response windows, alert intervals,marker channel timing, etc.

An electrode configuration switch 674 connects the sensing electronicsto the desired terminals 647-652 of corresponding sensing electrodes.For example, a portion of the terminals may be coupled to electrodesconfigured to define a sensing and/or shocking vector that passesthrough the left ventricle. The switch 674 may connect terminals to theevent marker sensing circuit 684 (which corresponds to the event markersensing channel) and the microcontroller. The circuit 684 may amplify,filter, digitize and/or otherwise process the sensed signals from theselect electrodes.

The switch 674 also connects various combinations of the electrodes toan impedance measuring circuit 613. The impedance measuring circuit 613includes inputs to collect multiple measured impedances betweencorresponding multiple combinations of electrodes. For example, theimpedance measuring circuit 613 may collect a measured impedance foreach or a subset of the active sensing vectors. Optionally, theimpedance measuring circuit 613 may measure respiration or minuteventilation; measure thoracic impedance for determining shockthresholds: detect when the device has been implanted; measure strokevolume; and detect the opening of heart valves, etc.

The switch bank 674 includes a plurality of switches for connecting thedesired electrodes to the appropriate I/O circuits, thereby providingcomplete electrode programmability. The switch 674, in response to acontrol signal from the microcontroller 660, determines the polarity ofthe stimulation pulses (e.g., unipolar, bipolar, co-bipolar, etc.) byselectively closing the appropriate combination of switches (notspecifically shown). The outputs of the cardiac signal and event markersensing circuits 682 and 684 are connected to the microcontroller 660which, in turn, is able to trigger or inhibit the pulse generators 670and 672, respectively. The sensing circuits 682 and 684, in turn,receive control signals from the microcontroller 660 for purposes ofcontrolling the gain, threshold, the polarization charge removalcircuitry (not shown), and the timing of any blocking circuitry (notshown).

Cardiac signals are also applied to the inputs of an analog-to-digital(A/D) data acquisition system 690. The data acquisition system 690 isconfigured to acquire cardiac signals, convert the raw analog data intoa digital signal, and store the digital signals for later processingand/or telemetric transmission to an external device. The dataacquisition system 690 samples cardiac signals across any pair ofdesired electrodes. The data acquisition system 690 may be coupled tothe microcontroller 660, or other detection circuitry, for detecting anevoked response from the heart in response to an applied stimulus,thereby aiding in the detection of “capture.” Capture occurs when anelectrical stimulus applied to the heart is of sufficient energy todepolarize the cardiac tissue, thereby causing the heart muscle tocontract.

The microcontroller 660 is further coupled to a memory 694 by a suitabledata/address bus 696. The memory 694 stores programmable operating,impedance measurements, impedance derivation and therapy-relatedparameters used by the microcontroller 660. The operating andtherapy-related parameters define, for example, pacing pulse amplitude,pulse duration, electrode polarity, rate, sensitivity, automaticfeatures, arrhythmia detection criteria, and the amplitude, wave shapeand vector of each stimulating pulse to be delivered to the patient'sheart within each respective tier of therapy.

The operating and therapy-related parameters may be non-invasivelyprogrammed into the memory 694 through a telemetry circuit 600 intelemetric communication with the external device, such as a programmer,trans-telephonic transceiver, or a diagnostic system analyzer. Thetelemetry circuit 600 is activated by the microcontroller 660 by acontrol signal. The telemetry circuit 600 advantageously allows data andstatus information relating to the operation of the device (as containedin the microcontroller 660 or memory 694) to be sent to an externaldevice 101 through an established communication link 603.

The device 610 may include a physiologic sensor 607 to adjust pacingstimulation rate according to the exercise state of the patient. Thephysiological sensor 607 may further be used to detect changes incardiac output, changes in the physiological condition of the heart, ordiurnal changes in activity (e.g., detecting sleep and wake states). Thebattery 611 provides operating power to all of the circuits.

It will be readily understood that the components of the embodiments asgenerally described and illustrated in the Figures herein, may bearranged and designed in a wide variety of different configurations inaddition to the described example embodiments. Thus, the following moredetailed description of the example embodiments, as represented in theFigures, is not intended to limit the scope of the embodiments, asclaimed, but is merely representative of example embodiments.

Reference throughout this specification to “one embodiment” or “anembodiment” (or the like) means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, appearances of the phrases “in oneembodiment” or “in an embodiment” or the like in various placesthroughout this specification are not necessarily all referring to thesame embodiment.

Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided to give athorough understanding of embodiments. One skilled in the relevant artwill recognize, however, that the various embodiments can be practicedwithout one or more of the specific details, or with other methods,components, materials, etc. In other instances, well-known structures,materials, or operations are not shown or described in detail to avoidobfuscation. The following description is intended only by way ofexample, and simply illustrates certain example embodiments.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventivesubject matter without departing from its scope. While the dimensionsand types of materials described herein are intended to define theparameters of the inventive subject matter, they are by no meanslimiting and are exemplary embodiments. Many other embodiments will beapparent to one of ordinary skill in the art upon reviewing the abovedescription. The scope of the inventive subject matter should,therefore, be determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein.” Moreover, in the following claims, the terms “first,”“second,” and “third,” etc. are used merely as labels, and are notintended to impose numerical requirements on their objects. Further, thelimitations of the following claims are not written inmeans-plus-function format and are not intended to be interpreted basedon 35 U.S.C. § 112(f) unless and until such claim limitations expresslyuse the phrase “means for” followed by a statement of function void offurther structure.

What is claimed is:
 1. A subcutaneous implantable medical device (SIMD),comprising: a pulse generator (PG) configured to be positionedsubcutaneously within a lateral region of a chest of a patient, the PGhaving a housing that includes a PG electrode, the PG having anelectronics module; and an elongated lead that is electrically coupledto the pulse generator, the elongated lead including a first electrodethat is configured to be positioned along a first parastemal regionproximate a stemum of the patient and a second electrode that isconfigured to be positioned at an anterior region of the patient,wherein the first and second electrodes are coupled to be electricallycommon with one another, wherein the electronics module is configured toprovide electrical shocks for antiarrhythmic therapy along at least oneshocking vector between the PG electrode and the first and secondelectrodes.
 2. The SIMD of claim 1, wherein the anterior regionrepresents a second parastemal region, the second electrode configuredto be oriented to extend in a common direction with the first electrodeand along a midline of the stemum, the first and second electrodesconfigured to be spaced apart by a distance of between 1.5 cm and 8 cmas measured in a direction transverse to a midline of the sternum. 3.The SIMD of claim 2, wherein the first and second electrodes areconfigured to be positioned along opposite sides of the sternum andseparated from one another by a predetermined distance.
 4. The SIMD ofclaim 2, wherein the first and second electrodes are configured to bepositioned on a common side of the sternum and separated from oneanother by a predetermined distance.
 5. The SIMD of claim 4, wherein atleast one of the first and second electrodes has an active length of7-11 cm and the predetermined distance separating the first and secondelectrodes is 6.0 to 8.5 cm.
 6. The SIMD of claim 1, wherein theanterior region is locate proximate to a lower end of the stemum, thesecond electrode configured to be oriented to extend in a non-paralleldirection relative to a direction of the first electrode, the secondelectrode located at relative to a midline of the sternum that isvertically below the first electrode, the first electrode having alength that is greater than a length of the second electrodes.
 7. TheSIMD of claim 6, wherein the length of the first electrode is 7.0 cm to11.0 cm, and the length of the second electrode is 5.0 cm to 6.5 cm. 8.The SIMD of claim 1, wherein the lead includes a lead body having a basesegment with a proximal end to be electrically coupled to a header ofthe PG, the lead including first and second distal branches at a distalend of the lead, the first and second electrodes provided on the firstand second distal branches, respectively.
 9. The SIMD of claim 8,wherein the lead body further comprises a Y-connector connecting thefirst and second distal branches to a distal end of the base segment.10. The SIMD of claim 1, wherein the pulse generator and first andsecond electrodes define a shocking vector that provides to adefibrillation threshold of at most 20 Joules.
 11. The SIMD of claim 1,wherein the pulse generator and first and second electrodes define ashocking vector there between that exhibits an impedance along theshocking vector that is no more than 64 ohms.
 12. The SIMD of claim 1,wherein the pulse generator and first and second electrodes define ashocking vector there between that exhibits an impedance along theshocking vector that is no more than 56.2 ohms.
 13. A method comprising:implanting a pulse generator (PG) within a lateral region of a chest ofa patient, the PG having a housing that includes a PG electrode; andimplanting at least one lead having first and second electrodes that arecoupled to be electrically common with one another, wherein the firstand second electrodes are elongated, the implanting comprisingpositioned the first electrode along a first parastemal region proximateto a sternum of the patient and positioning the second electrode at ananterior region of the patient; and configuring the electronics moduleto provide electrical shocks for antiarrhythmic therapy along at leastone shocking vector between the PG electrode and the first and secondelectrodes.
 14. The method of claim 13, further comprising positioningthe first and second electrodes at a dual parastemal position extendingin a common direction and spaced apart by a distance of 1.5 cm to 8.5 cm15. The method of claim 14, further comprising positioning the first andsecond electrodes along opposite sides of the sternum.
 16. The method ofclaim 14, further comprising positioning the first and second electrodeson a common side of the sternum.
 17. The method of claim 14, wherein atleast one of the first and second electrodes has an active length of8-11 cm and the predetermined distance separating the first and secondelectrodes is 6.0 to 8.5 cm.
 18. The method of claim 13, furthercomprising positioning the second electrode proximate to a lower end ofthe stemum and orienting the second electrode to extend in anon-parallel direction relative to a direction of the first electrode,locating the second electrode at a position, relative to a midline ofthe sternum, that is vertically below the first electrode.
 19. Themethod of claim 18, wherein the non-parallel direction orients alongitudinal axis of the second electrode perpendicular to alongitudinal axis of the first electrode.
 20. The method of claim 14,wherein the pulse generator is configured to generate a defibrillatingenergy of at most 40 Joules.